When does ovulation happen?

It's difficult to pinpoint exactly when ovulation happens, but usually it takes place 10-16 days before the start of your next period.

The first day of your period is day one of your menstrual cycle. The average cycle takes 28 days, but shorter or longer cycles are normal.

"It's not accurate to say that women are fertile on day 14 of the menstrual cycle," says Toni Belfield, a specialist in sexual health information and a trained fertility awareness teacher.

"If your cycle is 35 days long, you're not going to be fertile on day 14 of your cycle. You're more likely to be fertile around days 19-25. If you have a shorter cycle – for example, 23 days – you might ovulate around days 7-13."

Avoiding pregnancy

It's difficult to know exactly when ovulation happens. So if you're trying to avoid pregnancy, there isn't a "safe" time of the month to have unprotected sex.

"A woman who doesn't want to become pregnant shouldn't take the risk," says Belfield.

For a woman with a shorter menstrual cycle (for example, 23 days) having unprotected sex during her period could put her at risk of pregnancy.

"Sperm can hang around for seven days, and she might ovulate very soon after her period has finished."

Some women use natural family planning to plan or avoid pregnancy. "Working out your fertile time is very dependent on knowing your cycle and knowing your fertility indicators, which takes observation," says Belfield.